EP0168605B1 - Berührungsfreie Ermittlung der Verflüssigung bei schmelzbaren Stoffen - Google Patents

Berührungsfreie Ermittlung der Verflüssigung bei schmelzbaren Stoffen Download PDF

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Publication number
EP0168605B1
EP0168605B1 EP85106709A EP85106709A EP0168605B1 EP 0168605 B1 EP0168605 B1 EP 0168605B1 EP 85106709 A EP85106709 A EP 85106709A EP 85106709 A EP85106709 A EP 85106709A EP 0168605 B1 EP0168605 B1 EP 0168605B1
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European Patent Office
Prior art keywords
solder
change
monitoring
temperature
detecting
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Expired
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EP85106709A
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English (en)
French (fr)
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EP0168605A3 (en
EP0168605A2 (de
Inventor
S. Ashod Dostoomian
Riccardo Vanzetti
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VANZETTI SYSTEMS Inc
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VANZETTI SYSTEMS Inc
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Publication of EP0168605A3 publication Critical patent/EP0168605A3/en
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/03—Observing, e.g. monitoring, the workpiece
    • B23K26/032—Observing, e.g. monitoring, the workpiece using optical means
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K1/00—Soldering, e.g. brazing, or unsoldering
    • B23K1/005—Soldering by means of radiant energy
    • B23K1/0056—Soldering by means of radiant energy soldering by means of beams, e.g. lasers, E.B.
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/03—Observing, e.g. monitoring, the workpiece
    • B23K26/034—Observing the temperature of the workpiece
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
    • H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
    • H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
    • H05K3/3494—Heating methods for reflowing of solder

Definitions

  • This invention relates to a method and apparatus for determining the exact moment when a material is undergoing a change of state, e.g., in industrial or scientific processes.
  • the invention is more particularly directed to a method according to the preamble of appended claim 1 and to apparatus according to the preamble of claim 9.
  • the invention is especially applicable to the electronics manufacturing industry, particularly in the manufacture of printed circuit boards and the like where molten solder is applied at an electrical connection in order to form a permanent mechanical and electrical bond between two conductors.
  • molten solder is applied at an electrical connection in order to form a permanent mechanical and electrical bond between two conductors.
  • solder joints are individually hand-soldered; for large-scale production, an entire circuit board containing hundreds or thousands of solder- joints-to-be can be soldered in one step by wave- soldering or by reflow soldering. In the former, after certain preparatory steps, the board is passed over the surface of a molten solder bath where the solder is caused to adhere to local areas at the intended joints.
  • solder pads are formed at the desired locations by use of molten solder which is then allowed to solidify.
  • the desired electrical conductors are then placed in mechanical contact with their proper pads and the entire board is raised to the desired temperature either by radiant heating or by various other methods. Careful control is required of heating rates and temperatures.
  • a method of soldering chips with solder balls on a substrate is disclosed in US-A-4 278 867.
  • a laser beam of proper wavelength is directed upon the upper surface of a chip to be soldered, so as to heat the areas thereof above the solder balls, to bring the latter to or above their melting point.
  • the chips are directly heated by the laser beam and their temperature is monitored, while the solder elements per se are only heated indirectly.
  • the present invention is particularly addressed to the process of reflow soldering.
  • a particular problem in this process is that all solder joints on a given board do not always have the same amount of solder or of adjoining metal in contact with the solder. The result is that various solder joints will have different heat-input requirements, whereas standard radiant or convective heat-input methods will tend to overheat the smaller joints while underheating the larger ones.
  • This problem is partly overcome by yet another method of reflow soldering, notably vapor phase soldering, but this method is not yet in wide use and it carries other problems which remain to be solved.
  • the invention makes use of a well known physical phenomenon dealing with solids which are being heated to their melting points. During heating, the temperature of the solid continues to rise until melting begins. At this point, the further addition of heat causes no further temperature increase until the material is fully melted, after which the liquid will be observed to increase in temperature.
  • melting or "fusion” are used to describe the process in which matter is transformed from the solid to the liquid phase.
  • crystalline ones such as ice, many metals, and so forth
  • the transition occurs abruptly at a well defined temperature. This temperature is a constant for each material, provided that the pressure is not changed.
  • amorphous materials such as glass, various waxes and non-crystalline metals pass through a gradual change of state over a broader temperature range.
  • Our invention deals primarily with those materials which melt abruptly.
  • a similar phenomenon occurs when a liquid or a solid turns to gas or "vapor".
  • Heat energy is again used in separating molecules so that no external temperature increase is seen during vaporization.
  • the amount of heat needed per gram is again fixed. It is called the "latent heat of vaporization" and is not necessarily related to the latent heat of fusion for the same material.
  • the latent heat of fusion is 333420 J/Kg (80 calories per gram) and the latent heat of vaporization is 2.255 106 J/Kg (539 calories per gram), both measured at atmospheric pressure.
  • Figure 1 illustrates an idealized surface- temperature history of a solid which is melted with a constant rate of heat input and is then frozen with a constant heat withdrawal rate.
  • the sloping lines show a constant rate of surface temperature change; the level lines show heat entering or leaving the substance without causing a temperature change.
  • the upper two sloping lines represent the heating and cooling of the material in the liquid state. These may or may not have the same slope as the lower sloping lines, depending upon whether the specific heat of the material changes during the transition.
  • Figure 1 assumes that the thermal changes which occur with time do so instantly throughout the material; that is, there are no thermal gradients and all parts are at the same temperature at any one time. Melting and freezing thus occur uniformly throughout the sample.
  • This method of temperature measurement and its various forms are referred to by various names such as radiation thermometry, optical pyrometry, infrared radiometry, and so forth.
  • the advantages offered are non-contact sensing (which can be from a convenient distance), high response speed and minimal disturbance of the temperature and process being measured.
  • thermometry An important consideration in radiation thermometry is that all surfaces at a given temperature do not emit the same intensity of radiation. Certain surfaces, such as black ones, and certain surface configurations, such as cavities or flat but roughened surfaces, are better emitters than most light-colored or shiny flat surfaces. This property is described in terms of a quantity called "emissivity", which is measured on a scale from zero to one (or 0 to 100%). The lower value would be for an ideal non-emitter and the upper one is for a perfect emitter, neither of which cases actually occurs in nature although they can be approached quite closely.
  • Emissivity is important because it must be taken into account when one attempts to relate the measured surface radiation to the actual surface temperature. Quite often, the emissivity of a target surface is unknown or is not known to the accuracy desired, which leaves varying amounts of uncertainty in the resulting temperature values. This point is mentioned here as a prelude to our later discussion in which we indicate that the use of our invention does not require a knowledge of target emissivity, which is an important advantage. Indeed, if the actual melting point of the target material is already known from separate measurements, then the emissivity can be determined via our method.
  • Another physical principle which is used in a preferred embodiment of our method, concerns the use of radiant energy to heat a target surface while, at the same time, monitoring the surface temperature by measuring its infrared radiation.
  • the heating-while-measuring process poses the problem that, unless preventive measures are taken, radiation from the heat source can be reflected by the target surface directly into the radiation detector where it can be mistaken for thermal radiation from the target, leading to measurement errors.
  • the first above-named element of the present invention may comprise an automatic "XY" positioning table.
  • Such tables with computer controllers, are used widely throughout industry with far-ranging applications. Their function is to impart a prescribed motion to an item which is being processed in some manner or other.
  • An example would be a machining operation in which a block of metal is sequentially brought to various positions under one or several cutting tools where it is milled, drilled, tapped, etc., to form a complicated shape such as an automotive engine block.
  • Positioning tables range in size from a few square inches, such as are used in the automatic assembly of microscopic electronic parts, to many square feet as used in the garment industry for cutting stacks of cloth into intricate shapes to be made into wearing apparel.
  • tables can be made which move in steps of 2.5 10-5 m (0.001") or less and have a positioning repeatability of one-tenth that amount.
  • Typical table parameters which are envisaged for use with our invention would be:
  • Table travel From a few inches in the X and Y directions, for a small circuit board, to several feet in each direction for a multiplicity of larger boards;
  • Stepping resolution and repeatability From 2.5 - 10 -5 m (0.001") to 12.7 10 -5 m (0.005") for the former, for most applications, with a repeatability from one-tenth to one-half that of the stepping resolution;
  • Table speed For adjacent targets which are spaced, typically, 1.27.10 -3 m (0.050") to 2.54 . 10-3 m (0.100") apart, the table acceleration, speed and deceleration should be sufficient to allow from five to ten targets per second to be momentarily positioned, at rest, under a designated optical axis, assuming that no time is taken for target processing. In the actual case of reflow soldering, the table will halt for a finite time during the heating step.
  • the radiant power source i.e., the radiant power source
  • another art which has become highly developed in the past decade is that of using focused laser beams to provide intense local heating of various targets.
  • Such beams can be used for either for the removal of metal or ceramic, for example, from a part which is to be drilled, scribed, cut, and so forth, or merely to provide a temperature increase such as for melting, heat-treating, welding, and so forth.
  • Many types of lasers are commercially available for these applications.
  • beam-power level from a few watts to several thousand
  • wavelength band with single or multiple wavelengths covering the spectrum from ultraviolet through infrared
  • CW steady beam
  • Other parameters of interest to laser engineers include spatial mode structure (single vs. multiple), beam divergence, power output stability, beam diameter, longitudinal mode spacing, amplitude noise and ripple, tube lifetime, and many others.
  • the Nd:YAG neodymium-doped yttrium-aluminumgarnet
  • C0 2 carbon dioxide
  • Various considerations such as cost, lifetime, required power, etc., enter into the user's choice between these lasers.
  • Our invention envisages the use of a Nd:YAG laser with a beam power in the range of thirty to fifty watts, with a multi-mode spatial structure, operated CW (continuous wave) and with an optical system that can deliver between one and ten watts of beam power onto a typical solder-joint location.
  • the fiber is made typically from glass or a related material having high transparency at the laser wavelength.
  • a reasonable fiber diameter would be about 6.35 10 -4 m (0.025"). This is large enough so that the beam which emerges from the laser is conveniently focused onto one end face by conventional optical methods, and it is thin enough to provide mechanical flexibility. This allows the laser to be placed in an out-of-the-way location and also provides vibration isolation between the laser .and XY table whose "start/ stop" motion can be a vibration source.
  • the output end of the fiber serves as a secondary source which is imaged, via conventional optics, onto the target to be heated.
  • the secondary source may be apertured so as to provide a square, rectangular or other shape more suited to the shapes of non-disclike targets, such as the elongated lap joints which are used with flat-pack integrated circuits.
  • beam-spot elongation in the target plans can be achieved by the addition of a cylindrical lens element in conjunction with the focusing lens. Methods of doing so will be familiar to lens-system designers. Electromechanical means of rotating the cylindrical element through 90 degrees or other angles in order to accommodate randomly oriented lap joints will be familiar to mechanical designers.
  • optical radiation detection system to be used as the third above-named element of the invention
  • a variety of methods are available in the art, varying in temperature range, temperature resolution, response speed, cost and so forth.
  • a preferred form uses a lead sulfide detector with conventional glass optics, a radiation chopper, and suitable electronics, as known in the art.
  • Another preferred form at higher cost but having finer temperature resolution and being useful with targets at lower temperatures, uses infrared optics with a cryogenically cooled detector of indium antimonide.
  • the infrared optics comprises one or more lenses of infrared transmitting materials such as sapphire, arsenic trisulfide, zinc selenide or many others which are known.
  • reflecting optics may be used, as is often done in these cases.
  • a separate blackbody radiation source can be used to provide a reference temperature.
  • response speeds in the range from microseconds to a few milliseconds are easily achieved by suitable circuitry design. This is more than adequate to follow the warmup of a typical solder joint which is being reflowed because this process requires a great many milliseconds to occur. Such response speeds are equally attainable with other detectors as well.
  • the signal processing system may be of the analog or digital type.
  • its function is to identify irregularities in the heating curve of the solder joint, such irregularities accompanying the solid/liquid transition. Its manner of operation will be more fully explained through later reference to the drawings describing our preferred embodiment of the invention.
  • the recommended analog method of processing the infrared signals involves the use of the second time- derivative of the thermal signal and the fact that a non-zero second derivative signifies the presence of a departure from a constant heating rate.
  • a moment-by- moment computer evaluation of the first derivative, or slope, of the heating curve is carried out. Each new value is compared with the previous one, and a difference exceeding a predetermined threshold is taken to signify an irregularity in the heating curve.
  • FIG 3 is a general diagram of a digital version of the present invention.
  • the heating source is the Nd:YAG laser 10 whose output beam is focused by a glass lens 1 into an optical fiber 2 where it emerges from the other end with a given divergence angle. It is then incident upon glass lens 3 where it is rendered somewhat more parallel and proceeds to a dichroic mirror (or "beam splitter") 4 which reflects it downward upon an infrared transmitting lens 5.
  • the function of lens 5 is to focus the laser beam upon the target mounted on the XY table, and also to collect and to direct upward some of the thermal radiation which leaves the target as it becomes warmed. This radiation is at greater wavelengths than the YAG radiation, being typically in the range from about two to five pm.
  • the dichroic mirror 4 is specially designed as to be largely transparent to the longer wavelength region, so that most of the thermal radiation passes through it and into a second infrared transmitting lens 6 where it is focused upon the infrared detector.
  • Dichroic beam splitters of the type used here are available from many optical interference filter suppliers who provide filter-design and fabrication services.
  • the red-light beam from a helium neon (HeNe) laser 12 is added to it to render the focused spot visible. This is helpful during manual programming operations when the various solder-joint locations are being entered into the computer. This is done by use of table-control keys which move the table so that each solder joint in turn is located on the optical axis, whereupon its location is automatically entered into the computer when the operator presses another key.
  • HeNe helium neon
  • the infrared detector signal is preamplified and is digitized by an analog-to-digital (AID) converter 14 whereupon it enters the computer 16.
  • AID analog-to-digital
  • the computer is responsible for several control and data processing functions:
  • the computer communicates with the external world via a keyboard, a video display, a printer and, if appropriate, another computer.
  • solder joints which are amenable to being reflowed by laser beams, comprising a lap joint (shown in Figures 4A and 4B before and after reflow, respectively) and a leadless chip carrier (LCC) joint (shown in Figures 5A and 5B before and after reflow, respectively).
  • the integrated circuit IC
  • the substrate often by use of adhesive beneath the case, such that the contacting surfaces of its electrical leads rest on their respective solder pads, which are rigid.
  • the IC is held tightly against the board such that its electrical leads are under slight compression.
  • the contacting portions of the leads sink into the solder, as shown in Figure 4B, becoming partly or fully immersed and relieving the pressure stresses.
  • the IC is then secure when the resulting solder joints have cooled.
  • Figure 6 illustrates a preferred embodiment of a radiation thermometer incorporating a reflow- sensing capability using an analog method, as opposed to the digital method of Figure 3.
  • the detector 20 is of lead sulfide and is chopped at 400 Hz.
  • the resulting AC signal is amplified and is converted to a D.C. signal through an "amplifier- converter board” (ACB) 22, after which the signal is “linearized” or processed in circuit 24 so as to correspond to temperature values rather than to detector-output values. Thereupon it is further amplified in amplifier 26 and is directed to the reflow sensing circuit 28.
  • ACB amplifier- converter board
  • An emissivity control 30 at the ACB 22 is an amplifier gain adjustment which is used in order to take account of the emissivity of the solder.
  • FIG. 7 A detailed diagram of the reflow sensing circuit is shown in Figure 7.
  • the two electrical stages at the upper left, denoted by components Z7 and Z8, comprise a pair of electronic differentiators in series. They provide, at terminals 32 and 34, the first and second time derivatives of the signal entering at terminal 36. At left center in Figure 7 are shown the respective derivatives associated with a particular input waveform. The functions of the remainder of the circuit will be apparent to electrical engineers.
  • the various parts include comparators, "flip-flops" and so forth which are used for timing, thresholding, latching and other functions associated with the routine use of the system. For convenience, the operation of the circuitry of Figure 7 will be briefly described.
  • the input to differentiating amplifier Z7 Upon the application of heat to the solder, the input to differentiating amplifier Z7 will rise as shown in the upper waveform in Figure 7.
  • amplifier Z7 When the temperature rises to a level in the vicinity of the melting temperature of the solder, amplifier Z7 will provide a low-level output which will be provided to the inverting input of amplifier Z8 and to the non-inverting input of amplifier Z1.
  • the low-level input to the amplifier Z8 will result in a positive spike at the Z8 output, in response to which amplifier Z2 will provide a clock signal to the clock terminal of flip-flop Z6A. Due to the low-level signal present at the D input terminal of the flip-flop Z6A, the Q output of the flip-flop will remain low.
  • the temperature will begin to increase and the input signal at terminal 36 will correspondingly increase as shown in the upper waveform in Figure 7.
  • the output of amplifier Z7 will again fall to a low level to cause the outputs of amplifier Z1 and NAND gate Z3A to become low and high respectively.
  • the downward transition in the output of amplifier Z7 will also result in a positive spike in the output of amplifier Z8, thereby resulting in a further clock pulse provided by amplifier Z2 to the clock terminal of flip-flop Z6A.
  • the D input to the flip-flop Z6A will still be high, so that the clock signal will result in a high-level Q output.
  • the NAND gate Z4B With both inputs at a high levpl, the NAND gate Z4B will provide a low-level output, thereby reverse-biasing the diode CR2 and permitting the circuit Z5A to be reset by a low-level reset signal at a timing determined by the delay circuit R13, C13.
  • NAND gate Z4B will result in a high-level output from gate Z4C to the set terminal of flipflop Z6B, thereby activating transistor Q 4 and energizing the "solder liquid” LED and closing an appropriate relay.
  • Curve (a) in Figure 8 shows the various parts of Curve (a) in Figure 8 so that the reader may associate them with the various stages of solder reflow. This curve is idealized here, consisting of straight lines and sharp junctures. Curve (b) in Figure 8 shows the first derivative with positive polarity, the inverse of that in Figure 7. Also, the second- derivative spikes of Curve (c) in Figure 8 are shown idealized, being more pronounced than in the Figure 7 case.
  • Figure 9 is an approximation of a typical reflow curve for a solder joint, this represents the amplified and filtered detector signal which is to be sampled and digitized by A/ D converter and then examined in the computer in the preferred embodiment of the invention. Shown above the first portion of the curve is a sequence of hypothetical signal values which might result from such a sampling. Below the curve are shown the increments in the successive signal values.
  • the manner in which the computer identifies the moments of the start and end of reflow is a differential one in which it examines the sequence of increments. As the curve starts upward, it is noted that an increasing progression occurs in the values of the increments. In this progression, the computer subtracts each increment value from the succeeding value and recognizes that the difference is a positive number. As reflow begins and the slope of the curve starts to decrease, a sequence of increments will be found where the difference between successive increments is a negative number. At the first occurrence of this, the computer signals the start of reflow. It continues to observe negative differences until reflow is complete and the curve starts upward again. Upon recognizing the first positive change in the incremental values, the computer signals the completion of reflow.
  • the differential method is an approximate simulation of the analog method of taking derivatives, as described earlier.
  • the extent to which it is approximate is determined by the sampling time which is selected; as the sampling interval is decreased, the simulation becomes more exact.
  • the computer is instructed to ignore the first "upturn” in the curve after the laser shutter opens and normal warming begins; otherwise, this could be mistaken for the "end of reflow".
  • An important feature of the invention is that, once reflow is completed, the method allows one to continue to heat the molten solder to a precise temperature, safely above the reflow point, without one's having to enter into the computer an emissivity value for the solder. Instead, assuming that the composition and therefore the exact melting point of the solder are known, the computer is able to derive its own emissivity value once reflow has been completed. It does this by noting the radiometric signal value at the moment when reflow is complete. Thereupon it can either calculate (by algorithm) or look up (in a "table") the radiometric value which a standard blackbody radiation source would have at precisely the solder melting point.
  • the blackbody value By dividing the observed value by the blackbody value, it arrives at the emissivity value. Thereafter, it can convert radiometric values to actual temperature values while the molten-solder temperature continues to rise, and it can cause the laser shutter to close exactly when some predetermined temperature value above the melting point has been reached.
  • the user may wish simply to specify that, after reflow has occurred, the heating continue to be applied until the radiometric signal is some pre-established percentage above the melting-point value.
  • the computer can be made to recognize when solidification has occurred, in case it is desired not to disturb the molten solder by accelerating the XY table toward the next target position.
  • the fixed optical parts and movable table of Figure 3 can be replaced by a fixed table and movable optical parts.
  • a flexible, infrared transmitting optical fiber can be added between the infrared detector and the lens/dichroic mirror system so that the latter may be moved while the other system parts remain stationary.
  • the optical system can be inverted so that its axis is directed upward, or in any other direction, in case it is desired to test samples from some direction other than vertically downward.
  • the system can be configured so that the heat-injection axis and the infrared-detection axis are directed toward each other, instead of being coincident.
  • the aforesaid axes may be of arbitrary inclination to each other so that heat-injection and infrared-detection may be from arbitrary directions.
  • a second means of reflow detection will now be described, which is to be included within the scope of the invention.
  • This means makes use of a reflectance method by which a change in the physical properties of a solder surface is detected at the moment of solid-to-liquid transition.
  • This visible change is a matter of common experience to laser-beam solder-reflow technologists. Most often it occurs in the form of a change from a slightly granular solid surface to a perfectly glossy molten one. It also sometimes happens during melting that the surface contour will change. In either case, use can be made of a narrow beam of light directed at the surface and of a photodetector so positioned as to receive some portion of the light reflected from the surface.
  • the surface reflections Prior to melting, the surface reflections will remain unchanged and the detector signal will be constant. At the moment that the solid-to-liquid transition occurs at the surface, a discontinuity will be noted in the detector signal. Most often, this will be a signal increase due to the enhanced reflectance of the molten surface. Conversely, it can be a decrease due to a change in surface contour, and this will depend upon the optical geometry of the system.
  • Figures 10A and 10B illustrate a hypothetical solder joint before and after reflow, respectively.
  • the jagged-line contour in Figure 10A represents the crystalline surface of solid solder and shows that the reflected rays spread throughout a small angle, with some of them bypassing the detector. In the molten case, the reflection is more mirrorlike so that the detector receives more reflected power.
  • Figure 11 illustrates an embodiment of the reflectance method, using a single detector and being incorporated into the existing concept which was shown in Figure 3.
  • the rightward- proceeding YAG laser radiation impinges upon the first dichroic mirror 40 and is largely deflected downward to the target 42 where the absorbed portion causes heating and where the remainder is reflected in various directions. A portion of it proceeds upward through the lower lens and most of it is reflected leftward by the first dichroic mirror 40, where it serves no purpose.
  • the mirror 40 is slightly transparent to the 1.06- micrometer laser radiation and so a portion of this radiation proceeds upward through the upper lens 46 to the second dichroic mirror 48.
  • This mirror serves the purpose of separating the 1-pm radiation from the longer-wave thermal radiation which is emitted by the heated target.
  • the thermal radiation proceeds to the infrared detector 50, as in Figure 3, whereas the 1-pm radiation is diverted to a near-infrared detector 52, preferably of silicon, which serves as the reflectance sensor for reflow.
  • the present invention teaches two methods whereby simultaneous reflow can be brought about, one using time-sharing of the laser beam and the other using spatial division of the beam.
  • the laser beam is brought to the target locations successively and repeatedly by use of scanning mirrors or other known beam deviators. While the beam is in transit, the laser shutter is closed. It opens only when the target is reached and for a specified time period in which the beam spot "dwells" on the target. Each exposure therefore stores a finite amount of energy in the joint-to-be, with the amount dependent upon the product of the laser-beam power and the exposure duration. The beam then impinges upon the remaining targets in succession, during which interval the previous targets are drained of some of their thermal energy. However, with a laser beam of sufficient power, the application of repeated energy pulses will result in a net temperature increase at each target and, after a sufficient interval, reflow of all targets will be achieved.
  • the power pulses into each joint can be metered, under computer control, based upon the heating needs of each joint as determined by the thermal detection system which concurrently scans the joints with the laser beam. Metering may be carried out by control of either the pulse duration or the laser-beam power by means of various modulators which are in common use.
  • a laser beam might be divided into four beams of equal intensity and these could be directed to four cylindrical lenses or other "spot-to-line” converters, and four "lines” of radiation could be made to impinge upon the four rows of contacts at an LCC, for example.
  • the continuous lines of radiation could either be masked into spots, one for each contact, so that undesired heating of the substrate between contacts would be avoided, or the lines could be broken into spots by the focusing action of a row of small, conventional lenses used in conjunction with each cylindrical lens.

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  • Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Plasma & Fusion (AREA)
  • Investigating Or Analyzing Materials Using Thermal Means (AREA)
  • Electric Connection Of Electric Components To Printed Circuits (AREA)

Claims (17)

1. Verfahren zum Aufschmelzlöten, bei dem eine erste und eine zweite Komponente, die miteinander verlötet werden sollen, in Kontakt mit einer Menge eines Lötmittels in fester Phase gebracht werden, das Lötmittel auf eine Temperatur über seine Schmelztemperatur erhitzt wird, um das Lötmittel aus der festen in die flüssige Phase zu überführen, und man das Lötmittel sich abkühlen und verfestigen läßt, dadurch gekennzeichnet, daß ferner das Lötmittel während des Erhitzens kontaktfrei mit Instrumenten überwacht wird, um zu bestimmen, wann es geschmolzen ist, wobei in dieser Überwachungsstufe der Phasenwechsel des Lötmittels durch Bestimmen von entweder der Temperaturänderungsgeschwindigkeit des Lötmittels oder einer Änderung in den Eigenschaften der Oberflächenreflexion des Lötmittels optisch erfaßt wird, wobei diese beiden Änderungen oberhalb der Schmelztemperatur eintreten, sowie das Erhitzen entsprechend dem Wert der beobachteten Eigenschaft unterbrochen wird.
2. Verfahren nach Anspruch 1, worin die Überwachungsstufe folgendes beinhaltet: Erfassen eines Emissionsniveaus einer vorbestimmten Wellenlänge des von dem Lötmittel ausgehenden Lichts, wobei dieses Emissionsniveau von der Temperatur des Lötmittels abhängt, Erfassen einer Abnahme der Geschwindigkeit, bei der das genannte Emissionsniveau zunimmt, um dadurch den Beginn des Schmelzens des Lötmittels zu bestimmen, sowie Erfassen einer nachfolgenden Zunahme der Geschwindigkeit, bei der das genannte Emissionsniveau zunimmt, um dadurch festzustellen, daß das Lötmittel im wesentlichen vollständig geschmolzen ist.
3. Verfahren nach Anspruch 2, wobei ferner ein kontaktfreies Überwachen der Lötmitteltemperatur nach der genannten nachfolgenden Zunahme erfolgt, wobei die Erhitzungsstufe fortgesetzt wird, bis das Lötmittel eine Temperatur über der Schmelztemperatur erreicht hat.
4. Verfahren nach Anspruch 1, worin die Stufe des kontaktfreien Überwachens folgendes umfaßt: Ausrichten eines optischen Strahls auf das Lötmittel, um ihn an dem Lötmittel zu reflektieren, Erfassen der von dem Lötmittel zu einem vorbestimmten Erfassungspunkt reflektierten Lichtmenge sowie Beobachten einer Änderung in der erfaßten Menge des reflektierten Lichts, was als Ergebnis einer Änderung in der Oberflächenglätte des Lötmittels über dem Schmelzpunkt geschieht.
5. Verfahren nach Anspruch 1, worin der Schritt des kontaktlosen Überwachens folgendes umfaßt: Ausrichten eines optischen Strahls auf das Lötmittel, um eine Reflexion durch das Lötmittel hervorzurufen, Erfassen der Menge des von dem Lötmittel bis zu einem vorbestimmten Erfassungspunkt reflektierten Lichts und Überwachen einer Änderung in der erfaßten Menge des reflektierten Lichts, die als Ergebnis einer Änderung in der Oberflächenkontur des Lötmittels auftritt.
6. Verfahren nach Anspruch 4 oder 5, worin die Stufe des Erfassens der Menge des genannten reflektierten Lichts das gleichzeitige Erfassen von reflektiertem Licht an mindestens zwei verschiedenen Orten beinhaltet.
7. Verfahren nach Anspruch 1, worin der Schritt des Anordnens folgendes umfaßt: Anordnen einer Mehrzahl von ersten Komponenten und einer Mehrzahl von entsprechenden zweiten Komponenten in Kontakt mit entsprechenden Mengen an Lötmittel in fester Phase an einer Mehrzahl von Lötstellen, wobei jede Lötstelle eine der genannten ersten Komponenten, eine der genannten zweiten Komponenten sowie eine entsprechende Menge an Lötmittel vorliegen hat, wobei die Erhitzungsstufe das schrittweise Erhitzen der Mehrzahl der Lötstellen umfaßt, während die genannte Eigenschaft an jeder Lötstelle getrennt überwacht wird, und wobei das Zuführen von Wärme an jeder Stelle unterbrochen wird, wo das Lötmittel auf eine Temperatur oberhalb seiner Schmelztemperatur erhitzt worden ist, während das Zuführen von Wärme zu den übrigen Löstellen fortgesetzt wird.
8. Verfahren nach Anspruch 1, worin die Stufe des Anordnens das Anordnen einer Mehrzahl von ersten Komponenten und einer entsprechenden Mehrzahl von zweiten Komponenten in Kontakt mit entsprechenden Mengen an Lötmittel in fester Phase an einer entsprechenden Mehrzahl an Lötstellen umfaßt, wobei die entsprechenden Mengen an Lötmittel mindestens ein Lötmittel in einem Stück beinhaltet, das in Kontakt mit mindestens zwei der Lötstellen steht, und wobei die Stufe des Erhitzens das Erhitzen von mindestens zwei Lötstellen bis zum Schmelzen des Lötmittels und Trennen des Lötmittels in zwei individuelle Lötmittelmengen umfaßt.
9. Vorrichtung zum Aufschmelzlöten, wobei erste und zweite Komponenten im Kontakt mit einer Menge an Lötmittel in fester Phase angeordnet sind, wobei man das Lötmittel auf eine Temperatur über seiner Schmelztemperatur erhitzt und anschließend abkühlen läßt, dadurch gekennzeichnet, daß die Vorrichtung folgendes umfaßt:
- eine Erhitzungsvorrichtung (10) zum Aufbringen von Wärme auf das Lötmittel;
-eine Überwachungsvorrichtung (52) zum kontaktlosen Überwachen der Phasenänderung des Lötmittels durch Bestimmen entweder der Änderungsgeschwindigkeit der Temperatur des Lötmittels oder der Änderung in einer Eigenschaft der Oberflächenreflexion des Lötmittels, die beide oberhalb der Schmelztemperatur auftreten; und
-eine Unterbrechungsvorrichtung (18) zum Unterbrechen des Zuführens von Wärme zu dem Lötmittel gemäß dem Ausgangssignal der Überwachungsvorrichtung (52), wenn das Lötmittel einmal auf eine Temperatur über der Schmelztemperatur erhitzt worden ist.
10. Vorrichtung nach Anspruch 9, worin die Überwachungsvorrichtung folgendes umfaßt:
- einen ersten Generator (20) zum Erfassen des Emissionsniveaus einer vorbestimmten Wellenlänge des Lichts von dem Lötmittel und zum Erzeugen eines ersten Signals, welches das erfaßte Emissionsniveau anzeigt;
- einen zweiten Generator (Z7) zur Erzeugung eines zweiten Signals, das der Änderungsgeschwindigkeit des ersten Signals entspricht; und
- einen dritten Generator (Z8) zum Erzeugen eines dritten Signals, das der Änderungsgeschwindigkeit des zweiten Signals entspricht.
11. Vorrichtung nach Anspruch 10, worin das dritte Signal einen ersten Zustand (z.B. einen negativen) aufweist, wenn die genannte Änderungsgeschwindigkeit des ersten Signals abnimmt, und einen zweiten Zustand (z.B. einen positiven) aufweist, wenn die genannte Änderungsgeschwindigkeit des ersten Signals zunimmt, und die Unterbrechungsvorrichtung die Wärmezufuhr unterbricht, wenn ein erster Zustand des genannten dritten Signals auftritt, der von einem zweiten Zustand des genannten dritten Signals gefolgt wird.
12. Vorrichtung nach Anspruch 9, worin die Überwachungsvorrichtung folgendes umfaßt:
- einen ersten Generator zum Erzeugen eines analogen ersten Signals, welches das Emissionsniveau einer vorbestimmten Wellenlänge des Lichts von dem Lötmittel anzeigt;
- eine Vorrichtung (14) zum Ausmustern des ersten Signals, um eine Sequenz von ausgemusterten Signalwerten zu erhalten und diese in entsprechende digitale Signale umzuwandeln;
- eine Vorrichtung (16) zum Vergleichen von Werten von aufeinanderfolgenden digitalen Signalen, um eine Sequenz von Differenzwerten zu erhalten; und
- eine Vorrichtung (16) zum Vergleichen von aufeinanderfolgenden Differenzwerten, um zu bestimmen, ob die Änderungsgeschwindigkeit der Lötmitteltemperatur zu- oder abnimmt.
13. Vorrichtung nach Anspruch 12, worin die vorbestimmte Wellenlänge des Lichts eine Wellenlänge von Infrarotlicht ist.
14. Vorrichtung nach Anspruch 13, worin die Erhitzungsvorrichtung eine Einrichtung (12) aufweist, mit der optische Energie mit einer Wellenlänge, die von der genannten vorbestimmten Wellenlänge abweicht, auf das Lötmittel gerichtet werden kann.
15. Vorrichtung nach Anspruch 9, worin die Überwachungsvorrichtung folgendes umfaßt: Eine Vorrichtung (40, 44), um einen optischen Strahl auf das Lötmittel (42) zu richten und eine Reflexion an dem Lötmittel hervorzurufen; einen Detektor (52) zum Erfassen der Menge des bis zu einem vorbestimmten Punkt reflektierten Lichts; und eine Vorrichtung zum Überwachen einer Änderung in der erfaßten Menge des Reflexionsvermögens, die als Ergebnis einer Änderung in der Oberflächenglätte des Lötmittels (42) über dessen Schmelztemperatur auftritt.
16. Vorrichtung nach Anspruch 9, worin die Überwachungsvorrichtung eine Vorrichtung (40, 44), um damit einen optischen Strahl auf das Lötmittel (42) zu richten und eine Reflexion durch das Lötmittel hervorzurufen, einen Detektor (52) zum Erfassen der Menge des bis zu einem vorbestimmten Punkt reflektierten Lichts und eine Vorrichtung zum Überwachen einer Änderung in der erfaßten Menge des Reflexionsvermögens, die als Ergebnis einer Änderung in der Oberflächenkontur des Lötmittels (42) über der Schmelztemperatur auftritt, aufweist.
17. Vorrichtung nach Anspruch 15 oder 16, worin der Detektor eine Vorrichtung zum Erfassen der Menge des Lichts aufweist, das biz zu dem vorbestimmten Punkt und bis zu mindestens einem weiteren Punkt reflektiert wird.
EP85106709A 1984-06-11 1985-05-31 Berührungsfreie Ermittlung der Verflüssigung bei schmelzbaren Stoffen Expired EP0168605B1 (de)

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US06/619,438 US4657169A (en) 1984-06-11 1984-06-11 Non-contact detection of liquefaction in meltable materials

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US4657169A (en) 1987-04-14
JPS6110750A (ja) 1986-01-18

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